Add plugin docs

[ilevkivskyi]

This adds documentation for the plugin system. To avoid duplication, I add shorter and more basic info to the docs, while adding more technical details to the module/class/method docstrings.

To save time I would propose to make one-two rounds of review here and then polish the docs in subsequent PRs.

[ilevkivskyi]

Note that these docs already reflect changes in #6044

[JukkaL]

Thanks for documenting this! Left a bunch of comments. Feel free to leave some suggestions unaddressed for now -- but we should address them before the next release. There are some other minor things I can change myself once this has been merged.

[ilevkivskyi]

@JukkaL @msullivan I made practically all changes you requested. Please take a look once more and then I would propose to merge this.

[JukkaL]

Thanks for the updates! LGTM now. I'll prepare a PR that does some minor updates that are too inefficient to implement through code review.